The present invention relates to automotive radar sensors for adaptive cruise control and/or collision warning, for example, and more particularly to a method of detecting elevation mis-alignment of the radar sensor.
Automotive radar sensors typically have a limited field of view and are scanned in a horizontal (azimuth) plane to detect objects of interest. Although the elevation of a detected object is ordinarily not important, proper elevational alignment of the radar sensor is important to enable reliable identification of objects at relatively long range, and to facilitate rejection of overhead objects such as road signs and bridges. For this reason, radar sensors usually include an adjustment mechanism such as a screw that is rotated to change the elevation alignment of the sensor with respect to a vehicle mounting bracket.
The elevational alignment of a radar sensor may be determined either mechanically by using a bubble level or the like, or electronically by monitoring the emitted signal strength in the center of the elevation field of view or the return signal strength from a test target in the center of the elevation field of view. The electronic techniques are more amenable to factory or dealer calibration, in which case the elevation mechanism is adjusted either manually or automatically until maximum signal strength (emitted or returned) is achieved. A similar technique can be used for azimuth alignment, as described for example, in the U.S. Pat. No. 6,087,995 to Grace et al., where azimuth and elevation alignment are achieved simultaneously by adjusting the alignment to maximize a summation of the azimuth and elevation signals impinging on a set of interferometers aligned with the thrust axis of the vehicle. However, the sensitivity of the emitted or returned signal strength to changes in elevational alignment becomes very low as the sensor approaches the desired alignment, making precise and accurate alignment of the sensor very difficult to achieve. Moreover, many adjustments and measurements are usually required to optimize the alignment since no single measurement yields a reliable measure of the degree or direction of misalignment. Accordingly, what is needed is a method of determining the elevational misalignment of an automotive radar sensor with a single high sensitivity measurement.
The present invention is directed to an improved method of measuring elevational misalignment of an automotive radar sensor in a factory or service setting. According to the invention, two or more targets that can be discriminated by the radar system are positioned at different elevational angles with respect to the desired elevation angle, and the degree of elevational misalignment is determined according to the ratio or difference in return signal amplitude for the two targets. Discrimination of the targets may be ensured by Doppler (if the target is a transponder), or by differences in range or azimuth angle, provided of course, that signal strength variation due to the differences in range or azimuth angle are normalized. Since the amplitude difference is a measure of misalignment, the measurement may be used to verify proper alignment or to indicate the amount of adjustment required to achieve proper alignment.